The present invention relates in general to sewing machines and in particular to a new and useful monitor for indicating a low supply of thread in the sewing machine.
Devices are known for monitoring the supply of thread in a sewing machine equipped with a rotary hook for lock stitching, a light emitter which delivers light pulses to a light receiver. The light from the light emitter passes through apertures which are provided in the body of the rotary hook, the bobbin case, and the bobbin.
Such devices require a strong source of light, since the apertures in the bobbin and the hook body, provided for optically sensing low thread supply, are aligned with each other only for a very short period of time, especially at high sewing speeds, and the thread supply can be sensed only during this alignment. The time interval available for measuring the remaining thread supply is therefore extremely short and depends on the speed of the rotary hook and of the bobbin carrying the thread.
Phototransistors are always responsive to light incidence with a delay, due to their intrinsic properties. If the light pulse is shorter than the delay time, reduced signal amplitudes are obtained. The delay time becomes shorter with increasing intensity of the light. The light emitting diode employed as a source of light must therefore be operated at the limit of its loading capacity. In addition, with a relatively high current consumption, ample space is needed for the transformer furnishing the current so that the small space available in the sewing machine is still further reduced.